private static final long serialVersionUID = 0;
  }

  static <E extends @Nullable Object> Multiset<E> multiset(
      Multiset<E> multiset, @CheckForNull Object mutex) {
    if (multiset instanceof SynchronizedMultiset || multiset instanceof ImmutableMultiset) {
      return multiset;
    }
    return new SynchronizedMultiset<>(multiset, mutex);
  }

  static final class SynchronizedMultiset<E extends @Nullable Object>